1. Field of the Invention
The present invention is directed to metrology instruments employing a probe and, more particularly, relates to a method and apparatus for automatically and rapidly driving a probe of such an instrument into engagement with a sample.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a probe and a sample to obtain information concerning one or more characteristics of the sample. For example, scanning probe microscopes (SPMs) typically characterize the surface of a sample down to atomic dimensions by monitoring the interaction between the sample and a tip on the cantilever probe. By providing relative scanning movement between the probe and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The atomic force microscope (AFM) is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and which has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as strain gauges, capacitance sensors, etc. The probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Alternatively, some AFMs can at least selectively operate in an oscillation mode of operation such as TappingMode™. In TappingMode™ the tip is oscillated at or near a resonant frequency of the cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
One potentially problematic characteristic of SPMs (including AFMs) and other probe-based instruments lies in the technique employed to initially cause the probe to approach its “engaged” position. An engaged position is one in which the probe is located sufficiently close to the sample surface to obtain the desired measurement. This position varies with the characteristics of the instrument, the instrument's mode of operation, the sample, and the measurement being taken. The probe may be placed in this position by moving the probe toward the sample, the sample toward the probe, or a combination of both.
Because the probes are fragile, some amount of care is required to successfully engage a probe to the sample'surface. Typically, the operator starts with a large initial separation between the probe and sample to ensure that the probe and sample are not damaged when either needs to be exchanged. This initial separation is often several mm or greater, but may be as small as 25-50 um, depending on the system configuration, skill level and risk tolerance of the operator. From this initial separation, the operator typically manually reduces the probe-sample distance, often with the aid of an optical microscope to visually monitor the separation. The operator will typically position the tip to a secondary separation which is perhaps 20-1000 um from the surface, from which point an automatic engage algorithm is used for the final approach. This manual positioning step is time consuming and can be difficult for some operators. In many cases, depending on the skill of the operator and the configuration of the SPM, the manual positioning step can take several seconds to several minutes. Alternately, some systems use an external metrology device, such as an interferometer. Both systems are inexact and thus must allow for some margin of error, obviously leading to potentially longer engage times to accommodate the error.
From the secondary separation, the operator typically initiates an automatic engage algorithm which operates until the probe is engaged on the sample surface. During the automatic engage process, the system typically monitors one or more SPM feedback properties (for example, cantilever deflection or amplitude) for evidence of probe/sample interaction.
For the case in which the secondary separation is large such as 1000 microns, known automatic engage algorithms take a substantial amount of time to achieve the engaged position. Therefore, some techniques attempt to limit this final automatic engage movement to about 100 microns, and may be as little as 20 microns if sample clearance can be very closely controlled. The manual work required to achieve such secondary distances, however, typically offsets the benefits achieved with automatic engage times. A representative example of this operation is disclosed in U.S. Pat. No. 4,343,993 to Binnig, which is entitled “Scanning Tunneling Microscope” and the subject matter of which is disclosed herein by reference.
It is desirable to perform the entire engage process automatically and as quickly as possible. This goal is not easily met, however, because actuator precision and speed are, generally speaking, inversely related. Hence, a highly responsive, relatively low speed actuator is not well suited for rapidly closing the gap during the initial or “fast engage” phase of the engage process course. It also might not have sufficient range. More particularly, lack of range is one key reason for using two actuators. The Z piezo stage is actually capable of higher speeds than the SPM stage, but the SPM stage has about a 15,000 um range, while the Z Piezo Stage only has about a 8-16 um range (some manufacturers offer 50 um range Z Piezo scanners, but this is still too small). On the other hand, a less dynamically responsive, relatively high speed actuator that is capable of closing the probe/sample gap rapidly might not have sufficient smoothness/precision of motion to prevent the probe from slamming into the sample and damaging the probe and/or sample. For instance, if the Z-axis actuator diminishes probe/sample separation in insufficiently small increments, the approach may not be stopped before probe and/or sample are damaged by excessive interaction.
A proposed solution to this problem is disclosed U.S. Pat. No. 5,614,712, which is assigned to David J. Ray. The Ray patent introduces automatic coarse and fine position controls to a two-stage Z actuator. Referring to FIG. 1, the actuator 2 includes both a coarse positioning actuator 4, such as a stepper motor, and a fine positioning actuator 6, such as a piezoelectric actuator. The fine positioning actuator 6 is mounted on the coarse positioning actuator 4, and a probe 8 is mounted on the fine positioning actuator 6. Referring to FIG. 2, once tip-sample separation is blindly reduced a selected amount from a large distance (about 1000 microns) to a safe separation (about 100 microns, for example), the fine positioning actuator 6 is first extended to diminish the spacing between the probe 8 and the sample S while checking for probe/surface interaction indicative of engagement (compare position B to position A). If the fine positioning actuator extends through its full range without detecting the sample surface, it is retracted, as seen by comparing positions C and D with position B. The coarse positioning actuator 4 is then extended, typically incrementally, as seen in position A′, and the fine positioning actuator 6 is again extended while checking for probe/surface interaction indicative of engagement, as understood by those skilled in the AFM art. This “sewing” process is repeated as required until the fine positioning actuator drives the probe 8 into its engaged position as seen in position B′.
Engagement is thus constrained to occur during motion of the fine positioning actuator 6. This process is effective and automatic, but is relatively time consuming, often taking one minute or more to complete due in part to the fact that it requires extension and retraction of the fine positioning actuator with each incremental movement of the coarse positioning actuator. These slow engage speeds are tedious for all applications and unacceptable in applications in which a large number of measurements must be obtained as rapidly as possible. For instance, in many industrial applications employing step-and-repeat operations (semiconductor analysis, etc.), waiting for the probe to repeatedly re-engage can cut throughput in half or more.
Another two-stage actuator is disclosed in Japanese Patent No. 10311841 to IBM. The two-stage actuator disclosed in the Japanese IBM patent also has a coarse positioning actuator and a fine positioning actuator. However, unlike with the system disclosed in the Ray patent, transition between first and second stages is justified based on the difference in speeds of response rather than on the accuracy of positioning of the two stages. Hence, the engagement is constrained to occur during motion of a stage that has a relatively high-speed response but a limited range. As such, there is a greater risk that the tip and/or sample becomes damaged during the engage operation.
In all these systems the user is moving blindly through a selected distance in a continuous coarse positioning step. As a result, a system able to sense actual tip-sample separation would be ideal insofar as the continuous step could be used to move the initial tip-sample separation to a closer, yet still safe, separation.
In this regard, another limitation with known systems is that conventional SPM control architecture most often uses the host software to control the engage process. The host exists within the framework of the PC's operating system and therefore can be highly non-deterministic. Generally, during the engage process, the host alternates between sending commands to the SPM motion controller and the digital signal processor of the SPM over a PCI bus. Due to PCI bus latencies, it is not possible for the host to exercise real time control over the engage process, and therefore many of the actions have to be performed serially with added delays to accommodate these latencies. Possibly more importantly, prior to engage, known systems require the user to focus the corresponding optics of the SPM on the back of the probe, move the optics objective lens down by the desired probe-sample separation distance (e.g., 1000 microns), and then focus on the sample. In this case, notwithstanding the time required for each step, if either of these focal positions is off by more than more than a few tens of microns, the probe may be damaged.
Finally, as noted above, it is important to note that it is possible for the user to manually reduce the initial tip-sample separation to a secondary distance of less than 20 microns, and possibly even less than 10 microns, prior to performing the automatic portion of the engage process using, for example, one of the above-described methods. Such manual reduction may be performed once the probe is mounted in the AFM head, for example, by viewing the probe and sample as the two are brought into close range. From such safe distances, automatic engage times can be greatly reduced as compared to conventional methods. However, the relevant time in this case is the entire time required to bring the probe from the initial separation to the engaged position. In this case, it includes both the manual positioning step and the automatic engage time. For high throughput applications, the time and user intervention required to perform such a manual operation is prohibitive.
The need has therefore arisen to provide a feedback-controlled probe/surface engage of a probe-based instrument that is nondestructive to both the probe and the sample, automatic, and rapid, with generally no manual adjustment of the tip-sample separation (e.g., to a “safe” distance), to allow users to realize significant improvement in overall throughput. More specifically, a need exists to decrease AFM engage time from initial tip-sample separations (for example, exceeding 20 microns and preferably distances at or greater than 1 mm) from the current 15 to 25 seconds to less than three seconds while preserving probe/sample integrity, especially for sub-50 nm probes including critical dimension (CD) and focused ion beam (FIB) probes.